There is known a laser tracking interferometer which is configured from: a laser interferometer on which an optical axis shift detection sensor for detecting the amount of shift in the optical axis of return light is mounted; a biaxial rotation mechanism for directing the laser interferometer in an arbitrary direction; and a retroreflector that is secured to an object to be measured (see Japanese Patent Application Laid-Open. No. Sho. 63-231286 (hereafter referred to as Patent Literature 1) and Japanese Patent Application Laid-Open No. 2007-57522 (hereafter referred to as Patent Literature 2)). Here, the retroreflector is an optical element for collimating incident and reflected beams of light, and capable of interference measurement in a given direction by controlling the biaxial rotation mechanism so as to reduce the amount of shift in the optical axis to zero on the basis of an output from the optical axis shift detection sensor.
German Patent No. DE 102007004934 B4 (hereafter referred to as Patent Literature 3) describes a method for inspecting a geometric error of a positioning machine provided with a retroreflector using a laser tracking interferometer for measuring a distance by a laser beam that is rotatable to track the retroreflector, like those described above.
Referring to FIGS. 1 and 2, a description will be given of the technical outline of Patent Literature 3. Patent Literature 3 relates to a method for inspecting a geometric error of a positioning machine (herein, a CMM) 10 provided with a retroreflector 20 by a laser tracking interferometer 30 that measures a distance using a laser beam 32 rotatable to track the retroreflector 20.
FIG. 1 illustrates a base 12 of the positioning machine 10, a gate type frame 14 that is movable in the back-and-forth direction of the figure with respect to the base 12, a column 16 that is movable is the right-and-left direction along a horizontal beam 15 of the gate type frame 14, a slider 18 that is movable in the up-and-down direction with respect to the column 16, a personal computer (PC) 40, and communication cables 42 and 44. The tip end (the lower end in the figure) of the slider 18 is provided with a probe (for measurement) or the retroreflector 20 (for inspection).
As shown in FIGS. 1 and 2, claim 1 of Patent Literature 3 includes: a step of determining the coordinates of the rotation center M of the laser tracking interferometer 30 positioned in a work space of the positioning machine 10; a step of positioning the retroreflector 20 at at least two positions pi that are substantially positioned on a straight line gk extending through the rotation center M of the laser tracking interferometer 30, and detecting each of the coordinates of the retroreflector 20 by the positioning machine 10; a step of measuring, by the laser tracking interferometer 30, at least one distance difference Δdij,L between distances di from the at least two positions pi to the rotation center M, respectively; a step of computing at least one distance difference Δdij,C from each of the coordinates of the retroreflector 20 measured by the positioning machine 10; and a step of comparing the at least one measured distance difference Δdij,L with the at least one calculated distance difference Δdij,C wherein a step of computing each coordinate vector rM of the rotation center M includes:
(A) a step of moving the retroreflector 20 to a first position;
(B) a step of measuring the distance to the retroreflector 20 using the laser tracking interferometer 30;
(C) a step of repeatedly executing the steps (A) and (B) described above at other positions until the position (coordinate) vector rM of the rotation center M can be determined; and
(D) a step of computing each coordinate vector rM of the rotation center M from the measurement value of a measured distance.
Furthermore, claim 2 of Patent Literature 3 further specifies the method according to claim 1. The method is characterized in that the retroreflector 20 is moved to at least three positions and particularly, to four positions Pi, and the step (B) includes a step of using the laser tracking interferometer 30, when the retroreflector 20 is moved from one position Pi to another position Pi, to measure the distances from the rotation center M to the at least three positions Pi and measure a difference Δdij,L between the measurement values, so that each coordinate vector rM of the rotation center M is determined by each measured distance difference Δdij,L and the coordinate vector Pi measured by the positioning machine 10.